The present invention relates to the growth or removal of thin films, and more particularly concerns improved methods and apparatus for obtaining films of more precisely controlled thickness.
Thin film growth and etching are widely employed in many fields such as the manufacture of electronic circuits and components. Among other fields employing thin films is the entire field of optics where lenses and other optical components are coated with thin films to obtain various optical properties such as light transmission and reflection. Multi-layer non-reflective thin films or coatings are commonly used.
Film growth and etching process control typically requires the monitoring of a number of variables, such as temperature, pressure, gas flow rates and the like. Monitoring of such variables is performed by conventional sensors and controlled by known open and closed loop control methods to maintain repeatability. When the various parameters are precisely known and controlled, the desired film thickness is obtained by operating the process over a fixed period of time, assuming that the growth or etching rate remains constant with the constant parameters. However, sensor accuracy changes with time and, therefore, process repeatability is degraded. It is critically necessary to repetitively recalibrate the sensors in order to maintain satisfactory process performance. Further, sensitivity margins of some process variables may be lacking in some film growth and etching rates. These sensitivities include temperature effects on growth rate and effects of total area to be etched upon etching rate. Absence of such sensitivity margins makes the fixed time control methods less reliable.
To overcome these problems attempts have been made to employ various types of in-situ film thickness sensing in an effort to obtain direct endpoint control. Among several different methods of in-situ film thickness sensing is the technique known as ellipsometry. Ellipsometry, at present, is a well established method of thin film thickness measurement. By illuminating a sample with monochromatic light having a controlled state of polarization, and then analyzing the polarization state of the reflected light, ellipsometry gives direct access to optical constants of the surface under investigation. When the surface is covered by thin layers of film, both thickness and refractive index of the thin films can be obtained.
The state of polarization of light is defined by the phase and amplitude relationship between two component plane waves, that which is in the plane of incidence and that which is normal to the plane of incidence, into which the electric field oscillation is resolved. The effect of reflection of light from a surface is characterized by the angle delta, defined as the change in phase, and the angle psi, which is the arc tangent of the factor by which the amplitude ratio changes. Ellipsometry measures these quantities, delta and psi. From these quantities one can obtain optical constants of the reflecting material or the thickness and optical constants of the film, such as the index of refraction for transparent films and both index of refraction and the extinction coefficient for absorbing films. Ellipsometry is known to be sensitive to changes in film thickness down to one angstrom and is totally nondestructive. Recent development of automatic ellipsometers, which allow rapid sampling rates to monitor reaction dynamics, provides a basis for in-situ film thickness endpoint control.
Various techniques have been tried in attempts to use in-situ ellipsometry for endpoint control. It has been suggested to calculate a distance between a measured point and a set point to define an error. When the error is smallest, the objective has been reached. However, this method is very difficult to implement in real time. An alternative method, described more particularly below, calculates a variation value empirically or theoretically and compares this variation to the error to stop the process when the error is less than the selected variation. However, if the variation selected is too small the process may never be terminated, whereas if the selected variation is too large the process may be prematurely terminated.
End point control of film thickness is useful in many different types of film growth and etching process control systems and apparatus. One type of such film growth and etching processing is rapid thermal processing (frequently referred to as RTP). This type of processing is employed in the semiconductor industry for semiconductor wafer processing operations, such as implant, annealing, chemical vapor deposition (CVD) of dielectric and polycrystalline silicon films, silicide formation, and many others. Rapid thermal processing has many advantages over conventional batch furnace processing operations, including low thermal mass wafer heating and cooling using a lamp source, short thermal transients (fast heat up and cool down rates), small process chamber volume, and selective wafer heating for cold wall operation. It is compatible with real time, in-situ processing sensors, and is essential for some types of integrated processing.
In rapid thermal processing, system temperature ramp rates are typically on the order of 100.degree. C. per second, and thus optical pyrometry is most commonly employed for temperature control. However, even with extensive and careful calibration of temperature measuring instrumentation, the temperature measurement is subject to significant error. The external pyrometer is dependent for its measurement on surface emittance of the wafer which in turn depends on quality of the wafer as well as thickness of any growing film in a CVD process. Particularly for processing silicon wafers, normal error sources of the pyrometer include temperature dependence of the emissivity of silicon, and variations in wafer surface roughness. Accordingly, uncorrected pyrometer measurements can be in error by as much as 100.degree. C. in a CVD process. Where the RTP process employs a conventional fixed process time control, this temperature error can result in a changing deposition rate and thereby a large deviation in final film thickness. Various calibration schemes, attempting to compensate for variations in emissivity and wafer conditions, can improve reliability and accuracy of pyrometer measurements but have not yet provided a truly acceptable method for RTP process control with good process repeatability. The RTP process still is essentially open loop, depending upon a fixed time and maintenance of precise process parameters.
Accordingly, it is an object of the present invention to provide methods and apparatus which avoid or minimize the above mentioned problems.